1. Area of the Art
The invention relates generally to inhibition of tumor growth and specifically to inhibition of brain tumor growth.
2. Description of the Prior Art
Throughout this application various references are referred to within parentheses. Disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains. Full bibliographic citation for these references may be found at the end of this application, preceding the claims.
Brain tumors, like other solid tumors, require a perpetually increasing blood supply to maintain continuous growth beyond 1-2 mm3 (1,2). This is accomplished through angiogenosis, a process which occurs in response to endothelial growth factors released by tumor cells. Angiogenesis involves the induction of endothelial cell proliferation from quiescent microvasculature, migration of neoendothelium toward the tumor bed and, finally, maturation into a new capillary network (3). Brain tumors are the most angiogenic of all human neoplasias. The principal angiogenic factors demonstrated by either in situ hybridization or specific antibodies in tissue sections of patients with glioblastoma and medulloblastoma, the most common malignant brain tumors, are vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) (4-7). In fact, VEGF expression and microvessel density in glial tumors directly correlate with the degree of malignant characteristics and overall outcome (8-9).
Recent evidence suggests that angiogenesis is regulated by the activation of endothelial cell integrins, a family of transmembrane receptors which direct cell adhesion to extracellular matrix (ECM) proteins by binding to the amino acid sequence Arg-Gly-Asp (RGD)(10). In response to bFGF and VEGF, endothelial cells upregulate the expression of integrins xcex1vxcex23 and xcex1vxcex25 , respectively (11-13). Glioblastomas and their associated vascular endothelium have been found to express the integrins xcex1vxcex23 and xcex1vxcex25 (14,15). Integrin-mediated adhesion results in the propagation of intracellular signals which promote cell survival, proliferation, motility and capillary sprouting (16,17). Failure of these integrins to bind ligand results in endothelial cell apoptosis (18,19). The matrix glycoprotein, vitronectin, serves as ligand for xcex1vxcex23 and xcex1vxcex25 and is produced at the leading invasive edge by malignant gliomas (15,20). Together, these findings suggest a complex paracrine interaction between tumor cells, brain ECM and endothelial cell integrins for the continued angiogenesis and growth of malignant brain tumors.
Studies using the anti-xcex1vxcex23 antibody, LM-609, or an RGD cyclic peptide antagonist of xcex1vxcex23 and xcex1vxcex25 which prevents integrin-ECM interactions, have demonstrated an anti-angiogenesis response in the chicken chorioallantoic membrane (CAM) and a mouse chimera model (21-23). Other agents which act upon alternative sites of the angiogenesis pathway, such as antibodies to VEGF or its tyrosine kinase receptor fit, have also been effective in inhibiting angiogenesis (24,25).
Prior studies have shown that the attachment of breast carcinoma, melanoma and HT29-D4 colonic adenocarcinoma cells to vitronectin is dependent on xcex1v, xcex1vxcex23 and xcex1vxcex23, respectively (51-53). Vitronectin, which is produced by tumor and endothelial cells, is recognized by xcex1vxcex23 and xcex1vxcex25 and is an ECM protein found at sites of tumor invasion and neovascularization (54-55). Thus, in addition to supporting endothelial cell survival through xcex1v ligation, and hence angiogenesis, vitronectin expression may further enhance the adhesion of tumor and endothelial cells which express xcex1vxcex23 and xcex1vxcex25 integrins, thereby promoting their invasion. In one study using a SCID mouse/human chimeric model for breast cancer, tumor invasion was considerably reduced following the administration of the anti-xcex1vxcex23 antibody LM-609, suggesting a direct effect upon the tumor cell biology through a xcex1vxcex23 blockade (56).
Brain tumors, because of their highly invasive nature and degree of angiogenesis, afford an excellent model with which to further study the importance of integrins in tumor progression. Multiple studies have shown that microvessel density correlates with outcome and malignant grade in astrocytomas (57-59). Angiogenesis inhibitors, such as TNP-470, thrombospondinl and platelet factor 4, have been introduced into experimental brain tumors and have shown an inhibition of tumor growth (60-62). However, to date, no study has examined the effect of integrin antagonism on brain tumor invasion and angiogenesis. Therefore, a need exists to study the effect and thus provide a novel method for treating brain tumors.
The present invention is based on the surprise discovery that targeted antagonism of integrins, specifically xcex1vxcex23 and xcex1vxcex25, can substantially inhibit brain tumorigenesis in vivo. It is also based on the discovery that the microenvironment of the brain tumor is critical to the tumor behavior and in determining its responsiveness to such biologically directed therapies. The invention is further based on the discovery that integrin antagonism can have an anti-tumorigenic effect independent of anti-antiogenesis, which may act synergistically to retard tumor growth. For example, it is a discovery of the present invention that integrin antagonism may induce direct brain tumor cell death.
Accordingly, one aspect of the present invention provides a method of inhibiting tumor growth in the brain of a host. The method comprises administering to the host in need of such an inhibition a therapeutically effective amount of an antagonist of an integrin.
In one embodiment of the present invention, the integrin may be xcex1vxcex23 or xcex1vxcex23. The antagonist may be a polypeptide antagonist of xcex1v, an antibody against xcex1vxcex23, an antibody against xcex1vxcex25 or a combination of antibodies respectively against xcex1vxcex23 and xcex1vxcex25.
Another aspect of the present invention provides a method for inhibiting angiogenesis in a tumor tissue located in the brain of a host. The method comprises administering to the host a composition comprising an angiogenesis-inhibiting amount of an antagonist of an xcex1v integrin.
In one embodiment of the present invention, the intergrin is xcex1vxcex23 or xcex1vxcex25. The antagonist is a polypeptide antagonist of xcex1v, an antibody against xcex1vxcex23, an antibody against xcex1vxcex25 or a combination of antibodies respectively against xcex1vxcex23 and xcex1vxcex25.
A further aspect of the present invention provides a method of inhibiting ECM-dependent cell adhesion of brain tumor cells growing in the brain of a host. The method comprises administering to the host a therapeutic effective amount of an antagonist of an ax integrin, i.e., integrins xcex1vxcex23 or xcex1vxcex25. In one embodiment of the present invention, the antagonist is a polypeptide antagonist of xcex1v or a combination of antibodies respectively against xcex1vxcex23 and xcex1vxcex25.
Yet another aspect of the present invention provides a method of inhibiting vitronectin-dependent cell migration in brain tumor cells growing in the brain of a host. The method comprises administering to the host a therapeutically effective amount of an antagonist to as
In one embodiment of the present invention, the antagonist is a polypeptide antagonist of xcex1v or an antibody against xcex1vxcex23.
A further aspect of the present invention provides a method of inducing apoptosis in tumor cells growing in the brain of a host. The method comprises administering to the host a therapeutically effective amount of an antagonist of an integrin.
In one embodiment of the present invention, the integrin may be xcex1vxcex23 or xcex1vxcex25. The antagonist may be a polypeptide antagonist of xcex1v.
The methods of the present invention are well suited for use in treating brain tumors in vivo. The present invention provides a novel therapeutic approach to treat brain tumors.